1. Field of the Invention
The present invention relates to data network systems. In particular, the present invention relates to method and apparatus for providing transparently transporting 155 Mb/s signals through a high speed data network.
2. Description of the Related Art
Optical fiber is a transmission medium that is well suited to meet the increasing demand in data transmission in communication networks. Generally, optical fiber has a much greater bandwidth than metal-based transmission medium such as twisted copper pair or coaxial cable, and protocols such as the OC protocol have been developed for the transmission of data over optical fibers. Typical communications system based on optical fibers include a transmitter, an optical fiber, and a receiver. The transmitter converts the data to be transported into an optical form using the proper protocol and then transmits the resulting optical signal over the optical fiber to the receiver, where the original data is recovered from the optical signal.
While adding more optical fiber to the existing communications network infrastructure is a costly option to meet the increasing demand for communication bandwidth, in some locations, expanding the existing network may not be a viable alternative. Additionally, given the high cost of installation and the extensive amount of time required, adding more optical fiber is not always an attractive option to increase communications bandwidth.
Due to the large bandwidth capability of optical fibers, this type of transmission medium is most efficiently utilized when multiple users share the medium. In general, a number of low-speed data streams (“low speed channels”) transmitted by different users may be combined into a single high-speed channel for transporting across the optical fiber medium. At the opposite end of the communications network, when the high-speed channel reaches the destination for one of the low-speed channels that it is transporting, the low-speed channel must be extracted from the high-speed channel.
A typical optical communications network includes nodes (for example, central offices) which transmit high-speed channels to each other over optical fibers. In addition to transporting low-speed channels through the nodes (commonly referred to as the “pass-through” function) as part of high-speed channels passing through the nodes, nodes may also combine incoming low-speed channels to the high-speed channel (i.e., the “add” function) and/or extract outgoing low-speed channels from the high-speed channels (the “drop” function). These functions are commonly referred to as add-drop multiplexing (ADM).
For example, wavelength division multiplexing (WDM) and time division multiplexing (TDM) are two known approaches to combining low-speed channels into a high-speed channel. In WDM and its counterpart dense wavelength division multiplexing (DWDM), each low-speed channel is placed on an optical carrier of a different wavelength and the different wavelength carriers are combined to form the high-speed channel. Crosstalk between the low-speed channels is a significant concern in WDM, and thus the wavelengths for the optical carriers must be spaced sufficiently far apart (typically 50 GHz or more) so that the different low-speed channels are resolvable.
In TDM, each low-speed channel is compressed into a certain time slot and the time slots are then combined on a time basis to form the high-speed channel. For example, given a certain period of time, the high-speed channel may be capable of transporting 10 bits while each low-speed channel may only be capable of transmitting 1 bit. In this case, the first bit of the high-speed channel may be allocated to low-speed channel 1, the second bit to low-speed channel 2, and so on, thus forming a high-speed channel containing 10 low-speed channels. Generally, TDM requires precise synchronization of the different channels on a bit-by-bit basis (or byte-by-byte basis, in the case of SONET), and a memory buffer is typically also required to temporarily store data from the low-speed channels.